In a service routed network a network of service routers maintain aggregate routing information for inter-service-zone communication. These service routers exchange route prefixes and service router information to allow service routers to forward packets between a series of service zones. The service routers may be virtual, with each zone being supported by one or more appliances hosted on a virtual machine that provides the actual service. The service routers function as a bridge between inter-zonal communication and packet routing/switching through the appliance. An exemplary virtual service zone router executes in a virtual machine, however as an alternative, physical service zone routers may also be implemented. All service zones, their appliances, the virtual service routers and the routing of packets between the zones are called the service chain architecture.
Service chaining may be defined in terms of virtual routing and forwarding (VRF) and session routing and forwarding (SRF) functions. The VRFs are used for maintaining inter-zonal connectivity between service zones and list aggregate routes and the associated tunneling mechanisms for a service type, while the SRFs list which sessions (i.e., 5-tuple-flow) are served by what appliance in the zone. If a processing entity (e.g. data center blade) hosts multiple service types, an equal number of VRF/SRF combinations are available on the service router. The above-mentioned methods maintain VRF adjacency and the appliance session routes in SRFs, but fail to provide mechanisms for managing session state in SRFs.